1. Field of the Invention
The present invention generally relates to a speaker apparatus, and more particularly to an electrostatic speaker apparatus in which desired frequency versus sound pressure and vibration amplitude characteristics can be achieved.
2. Description of the Related Art
As known in the art, a so-called electrostatic (capacitor-type) speaker includes a pair of fixed electrodes that face each other, and a vibrator disposed between the pair of fixed electrodes. Such an electrostatic speaker is designed so that a driving signal is applied across the pair of fixed electrodes and a d.c. bias voltage is applied between an electrode of the vibrator and each of the fixed electrodes, thereby generating sound pressure according to the driving signal.
FIGS. 8 to 10 illustrate the structure of a conventional electrostatic speaker 1, by way of example. The electrostatic speaker (hereinafter referred to as xe2x80x9cspeaker apparatusxe2x80x9d) 1 includes frames 1a and 1b which are screwed to each other. Inside the frames 1a and 1b, fixed electrodes 2a and 2b are placed so as to face each other.
The fixed electrodes 2a and 2b are exposed to the air at openings 3a and 3b formed in the center of the frames 1a and 1b, respectively. As shown in FIG. 8, the fixed electrodes 2a and 2b are made of a substantially rectangular planar plate, and have multiple openings formed therein.
In the following description with respect to the structure, generally, the fixed electrodes 2a and 2b and a diaphragm 4 formed therebetween are rectangular; however, the fixed electrodes 2a and 2b and the diaphragm 4 may have a circular or any other shape.
The diaphragm 4 is placed in a gap between the fixed electrodes 2a and 2b. The outer periphery of the diaphragm 4 is held between a metal frame 5 and a vibrator electrode 6, and is fixedly received in the frames 1a and 1b through elastic members 7.
The diaphragm 4 has air gaps each having a predetermined length formed with respect to the fixed electrodes 2a and 2b. Specifically, as shown in FIG. 10, spacers 8a and 8b are disposed in close proximity to the fixed electrodes 2a and 2b, respectively, so that the diaphragm 4 is held between the spacers 8a and 8b. In this structure, the thickness of the spacers 8a and 8b can be used to precisely set the lengths of the air gaps, namely, a distance da from the fixed electrode 2a to the diaphragm 4 and a distance db from the fixed electrode 2b to the diaphragm 4.
Driving signals and d.c. bias voltages are applied to the speaker apparatus 1 having the above-described structure using a circuit shown in FIG. 11.
The circuit includes a booster transformer T1 and a transformer T2. A commercial power supply is connected to the primary winding of the booster transformer T1. The secondary winding of the booster transformer T1 is connected to a multistage voltage doubling rectifier circuit including diodes D1 to D7 and capacitors C1 to C8. The output of the multistage voltage doubling rectifier circuit is connected to the center tap of the secondary winding of the transformer T2.
The secondary winding of the transformer T2 is connected to the fixed electrodes 2a and 2b through resistors R3 and R4, respectively. One end of the secondary winding of the booster transformer T2 is connected to the vibrator electrode 6, or the diaphragm 4, through a resistor R1 and a terminal TC, so that d.c. bias voltages are applied between the diaphragm 4 and the fixed electrode 2a, and between the diaphragm 4 and the fixed electrode 2b. The d.c. bias voltages can be, for example, as high as 2.5 KV.
From a power amplifier connected to the speaker apparatus, audio signals are supplied across terminals which are connected to the primary winding of the transformer T2. As the audio signals are passed to the primary winding of the transformer T2 via a resistor R2, they are boosted by the transformer T2 before appearing on the secondary winding as the driving signals. The driving signals are thus applied to the fixed electrodes 2a and 2b through terminals TF and TR, respectively.
In the thus constructed electrostatic speaker 1, driving forces F are expressed by the following general equation (1) of the Coulomb""s law:                     F        =                  k          ·                                                    q                1                            ·                              q                2                                                    r              2                                                          (        1        )            
where q1 and q2 denote the charges of the electrodes, r denotes the distance between the electrodes, and k is the constant of proportion. Accordingly, the electrostatic speaker 1 is driven by the driving forces F calculated by general equation (1).
In the speaker apparatus 1, the fixed electrodes 2a and 2b are substantially rectangular, and the electrode surfaces facing the diaphragm 4 are entirely parallel to the diaphragm 4. In other words, the distance from any portion of the electrode surfaces to the diaphragm 4 is da or db shown in FIG. 10. Therefore, the driving forces applied to the diaphragm 4 are uniform over the diaphragm surface.
Now, a case is considered where uniform driving forces F which are expressed by F=F0sin(xcfx89t) are generated on the entire surface of the diaphragm 4 by the applied voltages. Then, the frequency characteristic of displacement distribution z across the diaphragm 4 is defined by the following equation (2):                     z        =                              ∑                          m              ,              n                                ⁢                                                    ∫                                                      (                                                                  F                        0                                            ⁢                                              Ξ                                                  m                          ,                          n                                                                                      )                                    ⁢                                      ⅆ                    V                                                                              M                ⁡                                  (                                                            ω                                              m                        ,                        n                                            2                                        -                                          ω                      2                                                        )                                                      ⁢                          Ξ                              m                ,                n                                      ⁢                          sin              ⁡                              (                                  ω                  ⁢                                      xe2x80x83                                    ⁢                  t                                )                                                                        (        2        )            
where (m, n) indicates the order of the unique vibration mode of the diaphragm 4, and "Xgr" denotes a reference function which indicates the displacement distribution in the unique vibration mode. Furthermore, V denotes the volume, M denotes the total weight of the diaphragm 4, and xcfx89m,n and xcfx89 denote the mnth-order resonance angular frequency and the angular frequency, respectively.
In the following description, the substantially rectangular diaphragm 4 has sides with lengths of a and b in the x- and y-axis directions, respectively, by way of example. The x-axis direction corresponds to the direction in which the longer sides extend, and the y-axis direction corresponds to the direction in which the shorter sides extend, as viewed from the surface direction of the diaphragm 4, as shown in FIG. 8. In this case, the displacement distribution in the unique vibration mode is given by the following equation (3):                               Ξ                      m            ,            n                          =                  sin          ⁢                      xe2x80x83                    ⁢                                    m              ⁢                              xe2x80x83                            ⁢              π                        a                    ⁢                      x            ·            sin                    ⁢                      xe2x80x83                    ⁢                                    n              ⁢                              xe2x80x83                            ⁢              π                        b                    ⁢          y                                    (        3        )            
When equation (3) is substituted into equation (2), the following equation (4) is derived with respect to the displacement distribution z across the diaphragm 4:                     z        =                              ∑                                                            m                  =                  1                                ,                3                ,                                  5                  ⁢                  …                                            ⁢                              
                            ⁢                                                n                  =                  1                                ,                3                ,                                  5                  ⁢                  …                                                              ⁢                                                    4                ⁢                                  F                  0                                ⁢                mn                ⁢                                  xe2x80x83                                ⁢                                  π                  2                                                            abM                ⁡                                  (                                                            ω                                              m                        ,                        n                                            2                                        -                                          ω                      2                                                        )                                                      ⁢                          Ξ                              m                ,                n                                      ⁢                          sin              ⁡                              (                                  ω                  ⁢                                      xe2x80x83                                    ⁢                  t                                )                                                                        (        4        )            
FIGS. 7A to 7D show four resonance modes as portions of the vibration modes indicated by equation (4).
FIG. 7A shows a resonance mode where (m, n)=(1, 1) in equation (4), showing the state where the entire surface of the diaphragm 4 vibrates in the same direction at a particular frequency. In a resonance mode where (m, n)=(3, 1) at a higher frequency, as shown in FIG. 7B, a vibration is created in a ridge-trough-ridge fashion along the longer sides (in the x-axis direction).
FIG. 7C shows a resonance mode where (m, n)=(1, 3) in which a vibration is created in a ridge-trough-ridge fashion along the shorter sides (in the y-axis direction). FIG. 7D shows a resonance mode where (m, n)=(3, 3) in which a vibration is created in a composite ridge-trough fashion along the longer sides and the shorter sides.
Therefore, the diaphragm 4 has a characteristic that multiple resonance modes may be generated, and the frequency characteristic found by calculation is shown in FIG. 12. FIG. 12 shows portions that may be affected by resonance in the resonance modes where (m, n)=(1, 1), (3, 1), (5, 1), and (7, 1).
Since multiple resonance modes may be generated, there have been problems with the speaker apparatus 1 in which the entire electrode surfaces of the fixed electrodes 2a and 2b are parallel to the diaphragm 4.
As shown in FIG. 12, there are peaks and dips in the amplitude versus frequency characteristic in the diaphragm center of the diaphragm 4 so that the characteristic is not smoothed. Many resonance modes are generated in the used frequency domain, resulting in peaks and dips in the output sound pressure versus frequency characteristic. Particularly, a problem occurs in the frequency domains corresponding to the resonance modes in which vibrations occur in reverse phase, as shown in FIGS. 7B, 7C, and 7D, in that the sound pressure level is reduced, namely, desired frequency versus sound pressure and vibration amplitude characteristics are not achieved.
In a similar way, in a speaker apparatus including fixed electrodes and a diaphragm which are shaped into a circle having a radius of a, the displacement distribution in the unique vibration mode across the diaphragm is expressed by the following equation (5) in the cylindrical coordinate system:
"Xgr"m,n=Jm(xcex2m,n,r).(Am,n,c cos mxcex8+Am,n,s sin mxcex8) for m=0, 1, 2, . . ., n=0, 1, 2,xe2x80x83xe2x80x83(5)
where Jm denotes an m-th Bessel function, r denotes the radial coordinate, A denotes the constant, xcex8 denotes the angular coordinate, and xcex2 is given by the following equation (6):
Jm(xcex2m,na)=0xe2x80x83xe2x80x83(6)
FIG. 13 shows vibration modes if uniform driving forces are acted on the entire surface of a circular diaphragm by the applied voltages in a similar way to the case where a rectangular diaphragm is used. In FIG. 13, m=0. Also in this case, a problem occurs in that multiple resonance modes are generated so that desired frequency versus sound pressure and vibration amplitude characteristics cannot be achieved.
A present inventor discloses an electrostatic speaker in Japanese Unexamined Patent Application Publication No. 9-182190 in which the surface shape of fixed electrodes is set so that distances from portions of the fixed electrodes to the surface of a vibrator vary and the driving forces applied to the vibrator are proportional to the surface displacement distribution in the unique vibration mode of the vibrator, whereby only a particular resonance mode is generated. For example, the surface shape of the fixed electrodes is curved so that distances from portions of the fixed electrodes to the surface of the vibrator are different and the driving forces applied to the vibrator are proportional to the surface displacement distribution in the unique vibration mode of the vibrator.
However, the electrostatic speaker disclosed in the above publication requires that the fixed electrodes be precisely machined, leading to a problem of an increase in cost.
Accordingly, it is an object of the present invention to provide a speaker apparatus in which only a single resonance mode is generated to achieve desired frequency versus sound pressure and vibration amplitude characteristics.
To this end, according to the present invention, a speaker apparatus includes a fixed electrode divided into a plurality of pieces, and a vibrator, which face each other. Driving signals of different voltages are applied to the divided fixed electrode pieces, and bias voltages are applied between an electrode of the vibrator and the fixed electrode, thereby applying driving forces to the vibrator according to the driving signals.
This allows distribution of the driving forces across the vibrator to be proportional to the surface displacement distribution in the unique vibration mode of the vibrator. Furthermore, there is no need to precisely machine the fixed electrodes, thereby providing a simplified driving circuit.